Task might leak in corner cases

[yibin87]

Bug Report

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It's detected when we enable random_tunnel_init_rpc_failure_failpoint. The whole story is like:

1. When Tiflash servers receive MPPTasks, they first do some preparation work and then register to TaskManager. And if TiDB's cancelMPPTask request came before thre registering, the task won't be cancelled. The task still have chances to fail itself if tunnel/receiver can pass the error message.
2. Since init_rpc failpoint will fail task very quickly, so it happened to reproduce such cases.

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